Various types of printers are known and used as standard output modules of computer systems. Among such printers is a printer apparatus of the bit-map controlled laser printer type which is now finding a growing range of practical applications as the output units of various data processing and computer graphics systems. This is because of the high-speed high-resolution printing capabilities and the ease of graphic data processing as can be achieved in a printer apparatus of the bit-map controlled laser printer type.
A laser printer has incorporated therein a character generator including a bit-map memory in addition to an ordinary font memory and has stored in the bit-map memory a collection of data required for the reproduction of images or character patterns for each page of printed output. The data thus stored in the bit-map memory are successively read out from the memory and are processed to produce digital signals carrying the graphic or character pattern data to be reproduced. A laser beam modulated with these digital signals is directed onto the peripheral surface of a photosensitive drum to produce thereon latent images corresponding to the graphic or character pattern data. These latent images are developed into visible toner images on the surface of the drum, from which the toner images are transferred to the surface of a print sheet
When an engineering drawing, a graphic representation of numerical data or a table of values is to be printed on a print sheet, it is frequently required to have character patterns printed in orientations or attitudes which differ from one character pattern or from one set of character patterns to another. Where there is such a requirement, one may use a word processor to enter the differently oriented character patterns and edit the image area while consulting the images of the character patterns indicated on the display screen. From the host computer of the word processor is thus supplied image and control data signals for the reproduction of the image containing the differently oriented character patterns. The laser type printer used for the word processor receives these image and control data signals to print the character patterns in the designated orientations as well as the graphic features on the surface of a print sheet in accordance with the signals received.
The signals to form differently oriented character patterns may be produced with use of a plurality of font memories having different character pattern orientations, respectively. The use of such a plurality of font memories is however not practically advisable because of the production cost required for the provision of the memories. A presently accepted approach is therefore to use a single front memory and transform the coordinate system of the data read from the memory into a coordinate system producing an angularly shifted version of the image represented by the data. The data read from the font memory are stored into the bit-map memory in the character generator of the printer at addresses designated for the transformation of the coordinate system of the character pattern data. Character pattern data representative of an angularly shifted version of the character pattern data is then read from the bit-map memory and the angularly shifted character patterns are then printed on the basis of the data thus read out from the bit-map memory.
This kind of approach however has problems encountered in formulaing the protocol between the printer and the host computer and in the management of the addresses in the bit-map memory.